Despite its dazzling location on the South Boston waterfront, the Fan Pier was starting to feel like a bedeviled backwater after Nicholas Pritzker, chairman of the Hyatt Development Corp., abandoned his plan for a hotel, office, and luxury housing complex there a few years ago. A bad turn in the real estate market that coincided with a family feud pushed Pritzker to sell. Now, both the commercial real estate market and the future of Fan Pier look a lot brighter. Local developer Joseph Fallon, who bought the property from Pritzker in 2005 for $115 million, broke ground yesterday for an 18-story office building.